This invention relates to an improvement of a so-called cog type V-belt which has notches or grooves in the lower surfaces. The invention is directed toward decreasing the noise to a lower level, which is generated during the run of the belt, by making the cogs of the belt different in pitch, angle and depth from one another.
A conventional cog type V-belt is higher in flexibility than an ordinary V-belt having no cogs. Accordingly, the cog type V-belt is advantageous in that less heat is generated during use, which leads to an increase of its service life. Also, it can be used with a small diameter pulley because of its high flexibility, and therefore it contributes to compact design.
However, the cog type V-belt is still disadvantageous in that it generates noise when operated, and therefore it is not suitable for a location where noise levels should be minimized. Accordingly, even through the cog type V-belt has the above-described advantages of increased power transmission performance, belt service life increases, and use with a small diameter pulley, it is not used as the V-belt in an automobile in which noises should be decreased as much as possible.
The cause of the generation of noises by the cog type V-belt is as follows: At the time that the cog type V-belt received in the pulley groove starts to move away from the pulley, the belt is positioned deep in the pulley groove, because of the belt tension normally applied thereto. Under this condition, the belt is forcibly released from the pulley. In the case of a cog belt, since the belt has grooves in the lower surface engaging the pulley, the belt is intermittently rather than continuously, released from the pulley. At the same time, the pulley side surfaces are wiped by the belt side surfaces, thus generating squeaky sounds. Since this operation is repeatedly carried out, noises result.
On the other hand, the ordinary belt having no grooves in the lower pulley engaging surface is continuously pulled out of the pulley groove, and therefore the volume of noises caused thereby is small.
The characteristic features of the noise caused during the belt drive operation are as follows:
(1) The volume of sound generated at a position where the belt leaves the pulley is a maximum.
(2) The volume of noise caused by the V-belt having no cog is small because the V-belt is continuously pulled out of the pulley. However, the volume of noise caused by the cog type V-belt is large, because the V-belt is intermittently pulled out of the pulley.
(3) If the coefficient of friction of the cog section is decreased, then the belt can be readily pulled out of the pulley and therefore the volume of noise can be made smaller. On the other hand, if the coefficient of friction of the cog section is increased, it becomes difficult to pull the belt out of the pulley, and accordingly the volume of noise is increased. Accordingly, the volume of noise caused by a raw edge type V-belt is larger than that of a wrapped belt having a low coefficient of friction.
(4) If a tension applied to the belt is increased, it becomes more difficult to pull the belt out of the pulley, and accordingly, the volume of noise is increased.
In the case of noise caused by the cog type belt, if the number of revolutions per minute of the driving pulley is increased under the condition that the tension of the belt is maintained unchanged, then the sound pressure level is abruptly increased when the number of revolution per minute reaches a certain value. If the number of revolutions per minute is further increased, then the sound pressure level is decreased initially, but thereafter it is gradually increased. Hereinafter, the sound pressure level abrupt increase at the particular r.p.m. of the driving pulley will be referred to as "a peak sound". In order to use the cog type V-belt in an automobile or a passenger car, it is essential to avoid the generation of the peak sound when the engine is running in the ordinary cruising speed range.
With the cog type V-belt, the r.p.m. with which the peak sound is generated depends on the pitch of the grooves formed in the lower surface of the belt. More specifically, when the groove pitch is increased, the r.p.m. with which the peak sound is generated is also increased. Thus, the r.p.m. with which the peak sound is generated is proportional to the pitch of the grooves formed in the lower surface of the belt. This means that the peak sound is generated when the number of times of pulling the belt cogs out of the pulley groove per unitary time becomes synchronized with the pulley.
In order to overcome the above-described difficulty, a cog type V-belt in which grooves are formed at random pitches in the lower surface of the belt, that is, a so-called random cog V-belt has been proposed as disclosed in Japanese Utility Model Application Publication No. Sho-52-117751. It has been considered therein that this random cog V-belt can decrease the peak sound because it is pulled out of the pulley groove unperiodically that is on a random basis; however, it has been found that in practice the peak sound is not decreased as much as expected merely by changing the cog pitch at random.
It has been found that in order to decrease the peak sound further, it is efficacious to design the belt so that the belt can be more readily pulled out of the pulley i.e., to decrease the frictional resistance to the belt being pulled out of the pulley.
In order to more readily pull the belt out of the pulley, it is efficacious (1) to increase the cog angle, (2) to decrease the cog depth, and/or (3) to decrease the hardness of the cog section. In order to decrease the frictional resistance which is caused when the belt is pulled out of the pulley, it is efficacious (1) to make the belt slippery by covering the cog section or the belt side surfaces with canvas, and (2) to decrease the coefficient of friction of the cog section.
In this invention, in order that the belt may be more readily pulled out of the pulley, grooves are formed in the lower surface of the belt in such a manner that at least two of the groove pitch (p) in the longitudinal direction of the belt, the groove depth (d) and the groove cut angle (a) are changed at random.
In this connection, it is well known in the art that, if the aforementioned groove cut angle is increased, then during the operation of the belt the contact area between the cog section and the pulley is decreased. Hence, the upper surface of the belt is deformed into a concave surface by the side pressure and as a result the belt is dropped into the pulley and the tension of the belt is decreased. If the cog depth is decreased, then the flexibility of the belt is decreased, and therefore the advantageous effects of the cog type belt are lowered.